This invention relates to a process and apparatus for the removal of hydrogen cyanide from gases containing other acidic components, such as carbon dioxide, hydrogen sulfide, and carbonyl sulfide, wherein the other acidic components are removed from the gases by a solvent scrubbing process with a regenerable solvent.
It is common for a wide variety of gases to contain acidic gaseous impurities. For example, gaseous fuels, e.g., coke oven gas, gases from coal gasification processes, gases from distillation operations, gases from partial oxidation operations and even natural gas contain, in addition to hydrocarbons, hydrogen sulfide, carbon dioxide, carbonyl sulfide (carbon oxysulfide), and frequently other components which are particularly troublesome during further processing, such as hydrogen cyanide in particular.
To remove these acidic components, it is known (DAS [German Published Application] 1,014,278) that the gas can be subjected to a solvent scrubbing step with a scrubbing agent which dissolves such components as hydrogen sulfide from the gas and is subsequently recycled into the process after regeneration. A special problem in this procedure is caused by hydrogen cyanide, on the one hand because it is readily soluble in numerous solvents and correspondingly strongly retained in the solvents and, on the other hand, because of its corrosive action and extraordinary toxicity. Therefore, the cited process provides, prior to the actual main scrubbing step which removes the hydrogen sulfide, a preliminary scrubbing process with alkaline-adjusted water in a separate preliminary scrubbing tower which serves to remove the hydrogen cyanide before all other gas components are scrubbed out. This is done, however, at a considerable expense, since the preliminary scrubbing column must be designed to accommodate the entire quantity of gas irrespective of the relatively minor amount of hydrogen cyanide contained in the gas. Furthermore, a scrubbing step with alkaline-adjusted water is ineffective with exhaust gases having high CO.sub.2 and H.sub.2 S partial pressures, since carbonic acid displaces hydrocyanic acid from KCN solutions.
For this reason, preliminary scrubbing steps with untreated water are more often utilized in practice. However, in these processes, the hydrocyanic acid is obtained as a highly dilute aqueous stream giving rise to either a serious water pollution problem or --when the scrubbing water is subsequently stripped with air or the like --a serious air pollution problem.